<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
      :root {
        --greeting: 'hello';
        --a-greeting: var(--greeting) ' world';
      }
      div:after {
        content: var(--a-greeting);
      }
    </style>
  </head>
  <body>
    <div>打个招呼吧！</div>
  </body>
</html>